1. Field of the invention
This invention relates to a semiconductor laser device which oscillates laser light with an oscillation wavelength in the visible region, especially, a high quality semiconductor laser device which can be readily produced by molecular beam epitaxy.
2. Description of the prior art
In recent years, single thin crystal film growth techniques such as molecular beam epitaxy (MBE) and metal organic-chemical vapor deposition (MO-CVD), have rapidly advanced. By these growth techniques, it is possible to obtain epitaxial growth layers of extreme thinness, on the order of 10 .ANG.. Due to the progress in these crystal growth techniques, it is possible to make laser devices based on device structures having very thin layers, which could not be easily manufactured by conventional liquid phase epitaxy. A typical example of these laser devices is the quantum well (QW) laser, in which the active layer has a thickness of 100 .ANG. or less resulting in the formation of quantum levels therein, whereas the active layer of the conventional double-heterostructure (DH) laser has a thickness of several hundreds of angstroms or more. Thus, this QW laser is advantageous over the conventional DH laser in that the threshold current level is reduced, the temperature characteristics are excellent, and the transient characteristics are excellent. This has been reported by W.T. Tsang, Applied Physics Letters, vol. 39, No. 10, pp. 786 (1981); N. K. Dutta, Journal of Applied Physics, vol. 53, No. 11, pp. 7211 (1982); and H. Iwamura, T. Saku, T. Ishibashi, K. Otsuka, Y. Horikoshi, Electronics Letters, vol. 19, No. 5, pp. 180 (1983).
As mentioned above, by the use of single thin crystal film growth techniques such as MBE and MO-CVD, it is now possible to put high quality semiconductor lasers having a new multiple-layered structure into practical use.
A typical structure for conventional QW lasers is the AlGaAs laser with a graded-index separate-confinement heterostructure (GRIN-SCH). The AlAs mole fraction (i.e., x) in an Al.sub.x Ga.sub.1-x As mixed crystal in the active region of visible-light GRIN-SCH semiconductor lasers with an Al.sub.0.3 Ga.sub.0.7 As mixed crystal quantum well is shown in FIG. 6. When the growth of the crystal layers of these laser devices is carried out by MBE, two Al cells must be used. One of the Al cells is opened so as to give an AlAs mole fraction of 0.3, and the other Al cell controls the cladding layers 3 and 7 and the GRIN regions 4 and 6. This is why two Al cells are required. Moreover, bulk AlGaAs is used as a quantum well layer, so that the radiation efficiency depending on the band structure of the bulk is decreased with the shortening of the wavelength, which causes difficulties in laser oscillation in the short-wavelength zone.